Respond to both GET and POST

According to [the docs] (http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/MakingRequests_MakingQueryRequestsArticle.html), SQS accepts both GET and POST requests. This small PR expands the existing `POST /` and `POST /:queue` handlers to do that. I also added a `GET /ping` endpoint to test whether the service is up, since now `GET /` is not a valid request (the Action parameter is required).